Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 02/17/21

Area Forecast Discussion...UPDATED

National Weather Service Chicago/Romeoville, IL

 1004 PM CST Tue Feb 16 2021

.UPDATE...
955 PM CST

Tweaks to the forecast have been continuing to maintain higher
lake effect snow chances into Porter County, IN through early
overnight, as well as extending clouds over some parts of
northeast Illinois longer and bumping up forecast lows there.

While lake effect snow (for this event) is nearing the end, it has
for most of the evening had a concentrated area of moderate snow
showers into Porter County. This has been well established on low-
level convergence and a presumed meso-low over the southern part
of the lake per analyzed wind streamlines on high-res guidance. As
has been the case, the lake effect parameters are not through the
roof (for instance equilibrium levels of only 6,000 ft), but
because of the very cold low-levels, the dendritic growth zone is
overlapped by the steep lapse rates and cloud depths. We suspect
some parts of Porter County have received around to a little over
2 inches this evening based on radar and webcam imagery, and could
finish with some nighttime totals of 3-4 in isolated spots of
northern Porter. Even if that were to happen, feel the nature of
this after the prolonged event is best handled with G-NOWs, a
Special Weather Statement, and other messaging.

Elsewhere, clouds from the lake effect fetch have meandered across
the south Chicago metro. Think these will stay due to diminishing
850-925 mb flow yet an inversion still in place above that (as
seen on 0230Z aircraft sounding data into MDW). The RAP seems to
be initializing this well and this solution keeps these clouds in
this area for several more hours before then moving them north as
the cloud-bearing blow becomes southerly.

Clouds are having a large influence on temperatures, not surprisingly
with a 1025 mb high moving overhead and fresh snow cover. Those
areas that are clear or can clear for several hours will be subzero,
and already have some of those readings including -6 in Rochelle.
Patchy shallow fog is certainly possible in parts of north
central ILlinois and in the far southern forecast area, though
confidence on that is low. Do not see that being a huge deal if it
develops, but isolated pockets of dense at just a few feet off the
ground is possible.

MTF

&&

.SHORT TERM...
229 PM CST

Through Wednesday...

The lake effect machine continues this afternoon, but the better
convergence signal has shifted out over the lake and into
northwest Indiana. There is still a loosely organized band with
occasional 25 dBz echoes, but the band is also fairly coherent
across much of the entire length of the lake. Therefore, these
have infrequently reduced visibility in LaPorte county to 1/2SM.
Forecast convergence/low level omega does show an uptick into
Porter county through about mid evening or so, and thus we are not
fully in the clear yet where some impactful lake effect snow may
occur, so use caution if traveling near the lake in northwest
Indiana through the evening.

For the rest of the area, expect cloud cover to slowly erode (where
it currently is now) through the evening as high pressure across the
plains/upper Midwest scoots on in. Light winds could lead to some
fog across north central IL and there is some guidance suggesting
this, but this is of low confidence and will need the evening shift
to take a bit closer look at this. With the light winds and fresh
snowpack, it will be a cold night with subzero temperatures. Some of
the colder guidance pushes -10 across our colder outlying areas of
north central IL.

The snow train continues later Wednesday, though maybe this part of
the ride will be a little less bumpy. Our next disturbance is back
across the Texas panhandle. This will spread toward the region later
Wednesday. This system track will place us on the less favorable
side of the upper jet, though a modest uptick in isentropic ascent
south of Chicago will lead to a generally lighter snow shield during
the afternoon. More limited ascent and drier air near Chicago
suggests the more widespread, though still lighter precipitation may
occur later Wednesday night into Thursday as some interaction with a
northern stream trough aids in better forcing for snow.

KMD

&&

.LONG TERM...
300 PM CST

Wednesday night through Tuesday...

As we head into the extended period the high amplitude trough
across the central CONUS will gradually begin to lift to the
northeast late in the week with a quick moving trough following
up behind for the weekend with a warming trend possible early next
week.

Although not as impactful as the previous 48 hours, light snow
remains in the forecast Wednesday evening into Thursday as a
weaker disturbance moves through the trough the area. There
remains considerable variability in the model guidance, similar
to how the past several low QPF snow events we dealt with the
past few weeks were often underdone. Because of this, felt
confident enough to maintain at least "chance" PoPs Wednesday
evening into the overnight hours with a follow up southern-stream
wave moving through. Some lake effect precip may still linger into
Thursday although some of the guidance has backed off on the more
bullish lake effect bands.

Heading into Friday, surface high pressure will set up shop over
the area which should help clear skies out a bit. This combined
with lighter winds and the deep snow pack will allow temperatures
to drop back into the negatives late Friday night into Saturday
morning. Although not as cold as last weekend, wind chill values
in the -5 to -15 range will be possible.

As the upper trough begins to lift east northeast the surface
high shifts into the eastern CONUS which will bring a return of
southerly flow to the area late Saturday into Sunday beginning a
potential warming trend into early next week. A more potent wave
moves through on Sunday with a surface low developing on the left
exit region of a strong upper jet. Model guidance remains somewhat
varied on the precip type for this event. However, with such a
deep snow pack still in place, suspect that the GFS guidance is
still too warm and prefer the ECMWF solution favoring mainly a
snow event for our area. For now keep the rain/snow mix line
mainly along and south of the Kankakee River valley.

Petr

&&

.AVIATION...
For the 00Z TAFs...

533 PM...Forecast concerns include...

Chance for light snow Wednesday afternoon into Wednesday evening.
Chance for light fog early Wednesday morning.

Lake effect snow showers continue into northwest IN, mainly east
of gyy though there have been a few flurries here at lot late
this afternoon along with some virga and carried flurries at gyy
for a few hours this evening. Otherwise, dry weather is expected
until Wednesday afternoon when there will be another chance for
light snow across the entire area. There remains some uncertainty
with this system as it will likely be weakening as it lifts across
the area. There are also some timing differences and opted to
carry tempo mention centered around 00z but some prevailing light
snow mention may be needed as trends emerge. If light snow
develops...vis in the 2sm range can be expected along with mvfr
cigs.

High pressure will move across the area with light westerly winds
likely becoming light and variable or calm tonight. Winds will
turn southerly Wednesday morning and southeasterly by afternoon.
The light winds and clearing skies may lead to some light/shallow
fog developing away from the Chicago metro area. Confidence if
very low and did not include any fog mention but its possible for
some fog to develop by morning, especially in the usual/rural
locations.

A bkn/ovc 4-5kft deck over the Chicago area is expected to slowly
shift southeast and dissipate this evening into the overnight
hours but low confidence for timing. A mid deck will spread across
the area Wednesday ahead of a lower, 3-4kft, vfr deck in the
afternoon. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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Area Forecast Discussion

National Weather Service Lubbock TX

 944 PM CST Tue Feb 16 2021

.UPDATE...
Low stratus is hanging on to most of the area where snow is not
falling and has prevented temperatures from falling. We believe
this trend will continue and as a result have increased the
overnight temperatures a few degrees where snow is not expected
to occur. Elsewhere, the winter weather advisory will be allowed
to continue through Wednesday morning. A narrow snow band was
setting up in the south-central to southeastern Texas Panhandle.
Latest CAM guidance indicates that the heaviest snow band will
remain north of the FA so a warning will not be issued at this
time.


&&

.PREV DISCUSSION... /issued 530 PM CST Tue Feb 16 2021/

AVIATION...
Light snow is coming to an end at KLBB and KPVW early this
evening. However, precipitation wrapping around a strong upper
level storm system will likely continue to bring snow to KCDS
through the night. Chances of additional snowfall at KLBB and KPVW
are not zero but they are low enough to not include in the TAF.
CIGS are now looking to stay in the MVFR range through the next
24 hours. Amd not scheduled at KCDS due to intermittent obs with
the METAR.

PREV DISCUSSION... /issued 249 PM CST Tue Feb 16 2021/

SHORT TERM...
A fast-moving 500 mb shortwave trough is pivoting across the CWA
this afternoon as it is embedded within a longer wave associated
with an upper-level low over south-central Canada. An intense 300 mb
jet streak approaching 150 knots is also digging into the Four
Corners region with a secondary vorticity maximum located over
central Arizona. At the surface, a stationary front (or pseudo-warm
front) is anchored across eastern New Mexico with a diffuse surface
trough associated with a weak, lee surface cyclone near the Raton
Mesa extending south-southeastward into the Texas Panhandle and
Edwards Plateau. This has been confirmed by an area of weak
confluence observed by the West Texas Mesonet near the I-27 and
Highway 87 corridors.

Mesoscale banding has developed across the forecast area, with
locally heavy snow rates per hour observed as an 850-700 mb warm
nose has advected into the Texas High Plains. The presence of
temperatures near -2 to -3 deg C beneath the planar crystal growth
zone (PCGZ) has resulted in some riming of snow, evident by both the
very large aggregates seen visually within the heavier bands and via
pilot reports over the last few hours. This has also been confirmed
via polarimetric Doppler radar imagery, with some slight
enhancements in ZDR and negligible reductions in rhoHV collocated
with higher reflectivity, indicating riming within the bands.

The snow bands embedded within the larger precipitation shield
should continue for a few more hours owing to the confluence of
strong Q-vectors associated with the forcing for ascent from the
shortwave and the thermally indirect, transverse circulation aloft.
The best Q-vector convergence should begin to shift to the east
later this afternoon, with snow coming to an end near sunset across
the southwestern South Plains while persisting across the northern
South Plains and Rolling Plains. These bands will be limited in both
spatial extent and time, and not every location will see
convectively-enhanced snow.

Overall, the synoptic-scale lifting is fairly weak and with a
dendritic growth zone that is just below supersaturation with
respect to ice, light snow is expected to remain the dominant form
of precipitation as dendritic growth has only been realized in the
more intense snow bands where tops to 10 kft MSL have been observed
by aircraft (thus favoring planar crystal/non-dendritic growth).
Additional snow accumulations of around one inch or less are
expected across the entire CWA through the evening hours, though
locally higher amounts will be possible within the aforementioned
mesoscale bands. Snow should taper off altogether early tomorrow
morning as the shortwave exits the region and the upper-level jet
axis moves overhead. Temperatures will once again crater into the
single digits tonight. Unnecessary travel is highly discouraged. If
you must travel a long distance tonight, pack a winter survival kit
in the event you become stuck along side of the road.

The low stratus deck will remain in place for the majority of the
day Wednesday before gradually lifting late in the afternoon, mainly
across the southwestern South Plains as the boundary-layer will
begin to mix out (albeit a very shallow layer to around 0.5 km AGL).
High temperatures as a result will only reach the middle 20s.
Otherwise, low clouds will return tomorrow night along with very
cold temperatures.

Sincavage

LONG TERM...
Latest in the series of impactful upper level low pressure troughs
will be swinging through the southern plains Wednesday night and
Thursday. Unlike the most recent troughs, this one appears more
starved of significant lift or deep moisture, though certainly
still will maintain a very chilly airmass across the area along
with a continued risk of some kind of low or mid level cloud
cover. So, we are maintaining cold conditions through Thursday but
also dry. There could be a risk for overnight and morning fog,
especially after this most recent snowfall but signals are
relatively unclear on these fog chances for now.

Friday into the weekend, the upper pattern will undergo
considerable flattening maintaining a solid westerly component
that will lead to a couple days of persistent surface low
pressure along the lee side of the southern and central Rockies.
The cold air entrenched across the area will slowly erode west to
east with upper 30s and 40s across the area Friday, then at least
another 10 degrees of warming expected Saturday finally returning
to near or even slightly warmer than normal at least on the
Caprock.

The next upper level trough will approach from the eastern Pacific
with much less amplitude though will bring a modest cold front
back through the area Sunday; timing issues between solutions are
unclear how much cooling might occur during the day Sunday, or if
it will wait until later in the day. Beyond that, solutions agree
on a return to a zonal-type westerly component aloft favoring
further warming and drying into early next week. We`ll take a look
at the possibility for additional buckling in the upper pattern
later next week in the days ahead. But in the meantime won`t argue
with above normal temperatures and dry weather for at least the
early to middle part of next week. RMcQueen

&&

.LUB WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 AM CST Wednesday for TXZ021>032.

&&

$$

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